An example of heat exchanger is disclosed in Patent Document 1. As shown in FIG. 93, the heat exchanger disclosed in this document includes a heat exchange coiled tube 40e placed in a housing 2e. The bottom of the space 3e surrounded by the coiled tube 40e is closed by a partition 6e. In the heat exchanger, when combustion gas is introduced from the top of the housing 2e, the combustion gas flows out from the space 3e by passing through the clearances of the coiled tube 40e and is discharged to the outside through the bottom opening of the housing 2e. On the other hand, a medium is supplied to the coiled tube 40e through one end thereof, and the medium is heated by the combustion gas. The heated medium flows out of the coiled tube 40e through the other end thereof.
With this structure, since the coiled tube 40e comprises a single helical tube, the structure is simple as compared with a heat exchanger utilizing a large number of finned tubes. Therefore, this structure is suitable for reducing the manufacturing cost and the size of the entire heat exchanger.
However, the above-described conventional structure has the following problems.
Since the heat exchange coiled tube 40e comprises a single helical tube, the degree of contact between the combustion gas introduced into the space 3e and the coiled tube 40e is small. Particularly, the degree of contact when the combustion gas passes through the clearances of the coiled tube 40e is small, so that the amount of heat transfer is small. Therefore, the heat exchange efficiency is low in the conventional structure. In recent years, for the purpose of environmental protection by fuel saving, reduction of the running cost and so on, the enhancement of the heat exchange efficiency of a heat exchanger is strongly needed. As effective means to enhance the heat exchange efficiency, it may be considered to recover latent heat from combustion gas (more precisely, latent heat of water vapor in combustion gas) in addition to sensible heat. However, with the conventional structure, it is difficult to recover such latent heat.
Patent Document 1: JP-U 61-69676